Class I Phosphatidylinositol 3-Kinases (PtdIns3Ks or PI3Ks) are heterodimeric lipid kinases consisting of a p110 catalytic subunit complexed to a regulatory subunit.
Class I of PI3Ks is composed of four isoforms which share a high degree of homology in the catalytic domain located towards their C-terminus. However, differences in the protein structure outside the catalytic domain and in the activation mechanisms allow to divide class I PI3Ks into class Ia and class Ib subclasses (Vanhaesebroeck et al. Proc. Natl. Acad. Sci., U.S.A., 1997; 94:4330-4335).
Class Ia enzymes consist of three distinct catalytic subunits (p110α, p110β and p110δ) that dimerise with five distinct regulatory subunits (p85α, p55α, p50α, p85β and p55γ) and all catalytic subunits are able to interact with all regulatory subunits to form a variety of heterodymers.
The unique member of class Ib PI3K (PI3Kγ) consists of a p110γ catalytic subunit that interacts with p101 and p87 regulatory subunits.
Furthermore, while all class Ia PI3K (PI3Kα, β and δ) enzymes are activated by tyrosine kinase receptors (RTKs), class Ib enzyme is triggered uniquely by G protein coupled receptors (GPCRs, Katso et al. Annu. Rev. Cell Dev. Biol. 2001; 14:615-675).
These enzymes phosphorylate phosphatidylinositol on the D-3 position of the inositol head group.
Phosphorylated forms of phosphatidylinositol are called phosphoinositides.
Although all class I PI3Ks are able to produce in vitro Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3), Phosphatidylinositol (3,4)-bisphosphate (PtdIns(3,4)P2) and Phosphatidylinositol (3)-phosphate (PtdIns(3)P), in vivo they are responsible for the production of PtdIns(3,4,5)P3 only.
The PtdIns(3,4,5)P3 metabolism is finely tuned and results from the balance between production and degradation. While PI3Ks regulate PtdIns(3,4,5)P3 production, the phosphatase PTEN degrades the PtdIns(3,4,5)P3 to PtdIns(4,5)P2 thus reducing the intracellular levels of the PtdIns(3,4,5)P3 and turning off the signaling cascade initiated by PI3K.
PtdIns(3,4,5)P3, generated by PI3Ks, acts as second messenger recruiting kinases with lipid binding domains (including plekstrin homology region, PH).
Major effectors of PI3Ks are the phosphoinositide-dependent kinase 1 (PDK1) and the serine-threonine protein kinase B/Akt which in turn mediates important biological effects of the PI3K pathway. Targets of Akt include transcription factors (e.g. FOXOs) and other kinases such as the mammalian target of rapamycin (mTOR) which regulates protein synthesis and cell growth.
Hence, when activated, class I PI3Ks, through PtdIns(3,4,5)P3 production, contribute to signaling cascades that influence cell proliferation and survival, insulin signaling, mitogenic responses and cell motility.
Being involved in cell proliferation and survival, the PI3K signaling pathway was found frequently altered in human cancer. Such alterations may occur upstream and/or downstream PI3K or may influence its lipid kinase activity (Hirsch et al. Pharmacol. Ther. 2008; 118:192-205).
Since RTKs activate PI3K, their over-expression or hyper-activation (increased kinase activity), often occurring during tumorigenesis, enhances the PI3K signaling which in turn results in increased cell proliferation and survival. As an example, it has been shown that in breast cancer over-expression of the ErbB2 cause hyper-activation of the PI3K signaling pathway (Fry M J. Breast Cancer Res. 2001; 3:304-312; Engelman J A. Nature Reviews Cancer 2009; 9: 550-562; Hirsch et al. Pharmacol. Ther. 2008; 118:192-205).
Alterations downstream PI3Ks often affect the phosphatase PTEN. During tumor development, mutations and deletions of PTEN inactivate its phosphatase activity thus increasing intracellular levels of PtdIns(3,4,5)P3 produced by PI3Ks. Genetic inactivation and loss of PTEN occurs frequently in many tumors such as glioblastoma, endometrial, prostate, lung and breast cancer (Bunney et al. Nat. Rev. Cancer 2010; 10: 342-352). Recent data have shown that, in a mouse model of prostate cancer, PTEN loss-driven tumorigenesis depends on PI3Kβ (Jia et al. Nature 2008; 454:776-779).
On the other hand, hyper-activating mutations and overexpression of PI3Ks cause increased PI3K lipid kinase activity with a consequent enhancement of PtdIns(3,4,5)P3 production. In this context PI3Ks are able to activate the downstream pathways even in the absence of stimulation by growth factors.
Consistent with this view, the gene encoding for PI3Kα (PIK3CA) is frequently mutated in human tumors. Somatic mutations of the PIK3CA gene have been reported in several cancer types including colon, ovary, breast, brain, liver, stomach, endometrial and lung cancer (Samuels et al. Science 2004; 304:554). Three hot-spot mutations (E542K, E545K, and H1047R) represent 80% of all PIK3CA mutations found in tumors.
To date, no genetic alterations have been found in the genes encoding for PI3Kβ, PI3Kγ and PI3Kδ.
Conversely, increased expression of PI3Kβ and PI3Kδ occurs in glioblastomas (Knobbe et al. Brain Pathol. 2003; 13:507-518), colon and bladder tumors (Benistant et al. Oncogene 2000; 19:5083-5090). Moreover, overexpression of wild-type PI3Kβ, PI3Kδ and PI3Kγ is sufficient to induce an oncogenic phenotype in cultured cells (Kang et al. Proc. Natl. Acad. Sci. USA 2006; 103:1289-1294). These findings suggest that PI3Kβ, PI3Kγ and PI3Kδ exert their oncogenic potential as wild-type proteins.
Several somatic mutations have also been reported for the gene that encodes for the regulatory subunit of Class Ia PI3K, p85α. Analysis of the mutations has revealed that these mutants lose their inhibitory activity on the p110 catalytic subunit, thus causing deregulated activation of all isoforms PI3Kα, PI3Kβ and PI3Kδ (Jaiswal et al. Cancer Cell 2009; 16:463-474).
The essential role of PI3K in human cancer made of this pathway an attractive target for molecularly targeted anticancer therapy. In the last years, academia and industry multiplied their efforts for the development of several pan-specific or isoform-specific PI3K inhibitors.
In addition to the role of PI3K in proliferative and survival signaling in tumors, class Ia PI3K may also mediate angiogenic events in endothelial cells in response to pro-angiogenetic factors such as the vascular endothelial growth factor (VEGF, Jiang et al. Adv Cancer Res. 2009; 102:19-65).
On the other hand, experimental observations have demonstrated that PI3K, and in particular PI3Kγ and PI3Kδ, are important mediators in the signaling cascade leading to the initiation of the inflammatory response. Given their central role in inflammation, both PI3Kγ and PI3Kδ have recently been investigated as new potential therapeutic targets for diseases caused by dysfunctional immune responses which include autoimmune disorders, allergic disorders, respiratory diseases and all pathologic conditions whose onset and/or progression is driven by an inflammatory insult, such as myocardial infarction and atherosclerosis (Ghigo et al. Bioessays 2010; 32:185-96).
In the light of these evidences, class I PI3K inhibitors might be useful in preventing inflammatory cell recruitment in a range of inflammatory and autoimmune diseases.
Moreover, since PI3Kγ and PI3Kβ are implicated in platelet aggregation, these enzymes have emerged as new targets in the treatment of thromboembolism (Hirsch et al. Thromb. Haemost. 2006; 95:29-35; Canobbio et al. Blood 2009; 114:2193-2196).